linux-kernel - Re: next-20250903 x86_64 clang-20 allyesconfig mmp_pdma.c:1188:14: error: shift count >= width of type [-Werror,-Wshift-count-overflow]

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Message-ID: <20250903165931.GA3288670@ax162>
Date: Wed, 3 Sep 2025 09:59:31 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Naresh Kamboju <naresh.kamboju@...aro.org>,
	clang-built-linux <llvm@...ts.linux.dev>,
	open list <linux-kernel@...r.kernel.org>, dmaengine@...r.kernel.org,
	lkft-triage@...ts.linaro.org,
	Linux Regressions <regressions@...ts.linux.dev>,
	Vinod Koul <vkoul@...nel.org>, Guodong Xu <guodong@...cstar.com>,
	Anders Roxell <anders.roxell@...aro.org>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Benjamin Copeland <benjamin.copeland@...aro.org>
Subject: Re: next-20250903 x86_64 clang-20 allyesconfig mmp_pdma.c:1188:14:
 error: shift count >= width of type [-Werror,-Wshift-count-overflow]

On Wed, Sep 03, 2025 at 02:04:10PM +0200, Arnd Bergmann wrote:
> On Wed, Sep 3, 2025, at 12:08, Naresh Kamboju wrote:
> 
> > Build error:
> > drivers/dma/mmp_pdma.c:1188:14: error: shift count >= width of type
> > [-Werror,-Wshift-count-overflow]
> >  1188 |         .dma_mask = DMA_BIT_MASK(64),   /* force 64-bit DMA
> > addr capability */
> >       |                     ^~~~~~~~~~~~~~~~
> > include/linux/dma-mapping.h:73:54: note: expanded from macro 'DMA_BIT_MASK'
> >    73 | #define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
> >       |                                                      ^ ~~~
> 
> I see two separate issues:
> 
> 1. The current DMA_BIT_MASK() definition seems unfortunate, as the
> '(n) == 64' check is meant to avoid this problem, but I think this
> only works inside of a function, not in a static structure definition.

Right, this is one of our longest outstanding issues :/

https://github.com/ClangBuiltLinux/linux/issues/92
https://github.com/llvm/llvm-project/issues/38137

This only happens at global scope.

> This could perhaps be avoided by replacing the ?: operator with
> __builtin_choose_expr(), but that likely causes other build failures.

Yeah, that makes the problem worse somehow even though GCC says the
non-taken option should not be evaluated...

  drivers/dma/mmp_pdma.c:1188:14: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
   1188 |         .dma_mask = DMA_BIT_MASK(64),   /* force 64-bit DMA addr capability */
        |                     ^~~~~~~~~~~~~~~~
  include/linux/dma-mapping.h:73:70: note: expanded from macro 'DMA_BIT_MASK'
     73 | #define DMA_BIT_MASK(n) __builtin_choose_expr((n) == 64, ~0ULL, (1ULL<<(n))-1)
        |                                                                      ^ ~~~
  drivers/dma/mmp_pdma.c:1323:27: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
   1323 |                 dma_set_mask(pdev->dev, DMA_BIT_MASK(64));
        |                                         ^~~~~~~~~~~~~~~~
  include/linux/dma-mapping.h:73:70: note: expanded from macro 'DMA_BIT_MASK'
     73 | #define DMA_BIT_MASK(n) __builtin_choose_expr((n) == 64, ~0ULL, (1ULL<<(n))-1)
        |                                                                      ^ ~~~

> Guodong, how about a patch to drop all the custom dma_mask handling
> and instead just use dma_set_mask_and_coherent(DMA_BIT_MASK(64))
> or dma_set_mask_and_coherent(DMA_BIT_MASK(32)) here? Instead of
> passing the mask in the mmp_pdma_ops, you can replace it e.g. with
> a 'bool addr64' flag, or an 'int dma_width' number that
> gets passed into the DMA_MASK_MASK().

If this works, I think it is worth pursuing to avoid this bogus
warning/error.

Cheers,
Nathan